Apparatus for operating electric discharge devices



Dec. 2, 1969 w. F. POWELL, JR 3,482,145

APPARATUS FOR OPERATING ELECTRIC DISCHARGE DEVICES Original Filed July23, 1962 1 NVEN TOR. Walter F. /%we// Jr;

United States Patent 3,482,145 APPARATUS FOR OPERATING ELECTRICDISCHARGE DEVICES Walter F. Powell, Jr., Danville, Ill., assignor toGeneral Electric Company, a corporation of New York Application June 16,1965, Ser. No. 464,517, now Patent No. 3,331,987, dated July 18, 1967,which is a division of application Ser. No. 211,554, July 23, 1962, nowPatent No. 3,249,799, dated May 3, 1966. Divided and this applicationJune 12, 1967, Ser. No. 645,737

Int. Cl. H05b 41/36 U.S. Cl. 315-206 5 Claims ABSTRACT OF THE DISCLOSUREApparatus employing a variable impedance network arrangement whichprovides an instantaneously varying impedance during a portion of eachhalf cycle to control the current supplied to a fluorescent lamp. Thenetwork includes at least one transistor connected in circuit with theoutput terminals of a full-wave bridge rectifier. A transformer isinterposed between the rectifier and lamp circuit with the primarywinding thereof connected in circuit with the collector electrode of thetransistor and the secondary thereof arranged for connection across thelamp. A current measuring resistor is connected between an outputterminal and the emitter of the transistor to influence the base driveof the transistor and thereby provide an instantaneously variableimpedance in the primary winding circuit of the transformer. Thevariable impedance in the primary winding circuit is used to regulatethe current flow in the secondary winding circuit and thereby effectregulation of the current supplied to the lamp.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a divisionalapplication of my copending application filed June 16, 1965, Ser. No.464,517 which issued July 18, 1967 as Patent No. 3,331,987, which inturn is a divisional application of the application copending therewithfiled July 23, 1962, Ser. No. 211,554 which issued May 3, 1966 as PatentNo. 3,249,799.

BACKGROUND OF THE INVENTION This invention relates generally toapparatus for operating electric discharge devices, such as fluorescentlamps, with alternating current. More particularly, it relates to animproved ballasting and operating arrangement for such apparatus.

The voltage required to initiate current flow in an electric dischargelamp varies with the length and type of electric discharge lampoperated. Usually the voltage required to operate the electric dischargelamp when normal lamp current is flowing through the lamp is less thanthe starting voltage. If the lamp current increases during operation,the voltage drop across the lamp will decrease as lamp currentincreases. This tendency of the lamp voltage to vary inversely with thelamp current is generally referred to as its negative resistancecharacteristic.

It is, therefore, a requirement of an apparatus for operating electricaldischarge lamps that it provide some means for limiting the currentsupplied to the lamp. If the current supplied to the lamp is not limitedby some means, the current will continue to build up until the lamp isdestroyed. A well-known way of limiting the current supplied to anelectric discharge device, such as a fluorescent lamp, is to provide aballasting resistor in series with the lamp.

The fluorescent lamp may be operated in a series loop arrangement whichincludes the ballasting resistor, the power source, and the lamp. Inorder to provide for stable operation and appropriate regulation of thelamp, the voltage drop across the ballasting resistor generally is aboutequal to the normal operating voltage of the fluorescent lamp. If thedifference between the starting voltage and the normal operating voltageof the lamp in such a resistive ballasting arrangement is small, slightchanges in the supply voltage would produce appreciable variations inthe light output of the lamp. It is, therefore, necessary inapplications where a resistor is used as a ballasting element to providea voltage drop across the ballasting resistor that is about equal to thenormal operating voltage of the lamp.

Where the fluorescent lamp is operated in a series loop arrangement witha ballasting resistor, it will be appreciated that the vector sum of thevoltage drop across the ballasting resistor and the voltage drop acrossthe lamp is equal to the supply voltage. Since the supply voltage isgenerally maintained at a substantially constant level, as the lampcurrent builds up because of the inherent negative resistancecharacteristic of the lamp, the current through the ballasting resistorincreases. This results in a proportional increase in the voltage dropacross the ballasting resistor thereby causing the voltage across thelamp to decrease. Conversely, when the lamp current decreases, thevoltage across the ballasting resistor decreases thereby causing thelamp voltage to increase. In this manner, the current supplied to thelamp is effectively limited.

Resistive elements have not been generally used in alternating currentballasting systems since they dissipate an appreciable amount of power.Reactive type of ballasting devices have been widely used since theyconsume less power than a ballasting resistor. Since reactive devices donot impede the flow of direct current, reactive ballasting elements havenot been used in direct current systems for ballasting. However,resistors have been used in direct current systems despite therelatively large power losses occurring in the resistor.

A principal disadvantage of conventional resistive ballasting systems isthat the power consumed by the ballasting resistor is generally aboutthe same as that required to operate the lamp. Thus, the efiiciency ofthe system is about fifty percent. It is desirable, therefore, to reducethe power losses in a resistive type of ballast while achievingsatisfactory regulation and stability. Further, it is desirable toprovide an apparatus for operating electric discharge lamps that doesnot require a large difference between the lamp starting voltage (opencircuit voltage) and the lamp operating voltage. It will be appreciatedthat as the difference between the starting voltage and the operatingvoltage is reduced, less energy is required to be dissipated or storedin the ballasting elements. Consequently, the components in the systemcan be smaller in size and weight, and where a ballasting resistor isemployed, less power is dissipated in the resistor.

Accordingly, it is a general object of the present invention to providean improved apparatus for operating electric discharge devices.

A more specific object of the present invention is to provide animproved apparatus for operating electric discharge lamps, such asfluorescent lamps, wherein the lamp can be operated with a relativelysmaller difference between the lamp starting voltage and the lampoperating voltage.

It is another object of the present invention to provide an improvedapparatus for operating a fluorescent lamp that utilizes a resistivetype of ballasting and can be operated at relatively greater efficiencythan conventional ballasting systems employing resistors as ballastingelements.

SUMMARY OF THE INVENTION In accordance with one form of my invention, Ihave provided an improved apparatus for operating at least one electricdischarge lamp, such as a fluorescent la-mp, from an alternating powersource that employs a variable impedance network arrangement. Thenetwork arrangement provides an instantaneously varying impedance duringa portion of each half cycle to control the current supplied to theelectric discharge lamp in order to prevent the lamp from destroyingitself because of its negative resistance characteristic. In thepreferred form of my invention, the variable impedance network includesat least one transistor that is driven to provide an instantaneouslyvariable impedance to control the lamp current, and a relatively lowimpedance is provided during an early and late portion of each halfcycle. The emitter and collector electrodes are connected in circuitwith the output terminals of a full-wave bridge rectifier. Base drivefor the transistor may be obtained from the full-wave bridge rectifieror may be obtained from a separate source, such as a feedback source, avariable D.C. supply or a fixed D.C. supply. The input terminals of thebridge rectifier may be placed directly in the lamp circuit, or if it isdesired to employ transistors having relatively lower voltage ratings, atransformer may be interposed between the bridge rectifier and the lampcircuit.

In another form of my invention, I have provided a variable impedancebridge network for controlling the current supplied to a fluorescentlamp which is comprised of a full-wave rectifier, a pair of transistors,and a transformer. One of the windings of the transformer is connectedin circuit with at least one output lead of the apparatus to place thevariable impedance bridge network in series circuit with the lamp duringoperation. The other of the transformer windings is connected in circuitwith the collector electrodes of the transistors and has a tap connectedin circuit with at least one output terminal of the full-wave rectifier.Further, a resistor may be connected in circuit with the other of theoutput terminals and in circuit with the emitter electrodes of thetransistors to function as a current measuring element. A bias supplymeans is connected in circuit with the base electrodes of thetransistors. The transistors are driven by the bias supply means toprovide an instantaneously variable impedance in the primary circuit ofthe transformer whereby the current supplied to the lamp is regulated.

According to another aspect of the invention, the bias supply means iscomprised of a transformer having a primary and a center tappedsecondary winding. The primary winding is connected to a suitable signalsource. For example, the transformer may be connected across the outputleads of the apparatus where it is desired to sense the voltage or inseries with the lamp where it is desired to sense the lamp current or toa separate source having a predetermined wave shape where it is desiredto provide a lamp current with a corresponding wave shape. Further, thecenter tap of the secondary winding is connected to one of the outputleads of the full-wave bridge rectifier, and the ends of the secondarywinding are connected to the base electrodes of the pair of transistorsto supply base drive current thereto.

The subject matter which I regard as my invention is set forth in theappended claims. The invention itself, however, together with otherobjects and advantages may be better understood by referring to thefollowing description taken in conjunction with the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING The figure of the drawing is aschematic circuit diagram of an apparatus embodying a form of myinvention wherein the variable impedance bridge network supplies thevoltage to the primary winding of a transformer for operating afluorescent lamp.

In the figure of the drawing, 1 have shown one form of my inventionembodying a variable impedance net work for controlling and supplyingthe current required for operation of an electric discharge lamp 99. Theapparatus for operating the electric discharge lamp 99 is generallyidentified by reference numeral 100 and is shown enclosed in a dashedrectangle 101. The apparatus 100 is energized by connecting a pair ofinput leads 102 and 103 across a suitable alternating current source.The output of the apparatus 100 is supplied to the electric dischargelamp 99 by output leads 104 and 105.

As will hereinafter be more fully explained, the variable impedancebridge network arrangement in the exemplification of the invention shownwill reproduce across output leads 104, 105 a current corresponding inwaveshape to the waveshape of a feedback signal applied across a pair offeedback leads 106, 107 or, in other words, across the primary winding Pof the bias signal transformer T The bias signal transformer T has apair of secondary windings S and S inductively coupled with the primarywinding P on a magnetic core 108.

It will be noted that the input leads 102 and 103 are connected with theinput terminals of a full wave bridge rectifier 109 which includesdiodes D D D and D One of the output terminals of the bridge rectifier109 is connected by lead 110 to the tap to which primary windings P andP of transformer T are joined. The other output terminal of bridgerectifier 109 is connected in circuit with the emitter electrodes oftransistors Q and Q through a resistor R and leads 111, 112 and 113 andis also connected with secondary windings S S by lead 114.

Continuing with the description of apparatus 100, the operation will nowbe more fully described. In order to start the operation of theapparatus 100, the input terminal leads 102 and 103 were connected to anAC. power source and the feedback leads 106 and 107 were also connectedwith an AC. power supply through a small filament transformer T to applya sinusoidal signal across the feedback leads 106 and 107.

Let us arbitrarily assume that the voltage across the primary winding Pat a given instant is such that the upper end of the winding P isnegative with respect to the lower end. As a result, the voltage inducedacross the secondary windings S and S is such that the upper end isnegative with respect to the lower end. A negative voltage is nowapplied at the base electrode of the transistor Q, to switch transistorQ into conduction. At this instant, substantially the entire outputvoltage of the bridge rectifier 109 is applied across primary P and avoltage is induced across the secondary winding S of the transformer TAssuming that this instantaneous voltage is sufiicient to ionize lamp99, lamp 99 will begin to conduct. Consequently, current begins to flowin the loop which includes lamp 99, output lead 104, secondary Winding Sand output lead 105.

A current flow through the secondary winding S in effect, lowers theresistance reflected to the primary winding P Consequently, more currentis supplied by the power source through the bridge rectifier 109.However, the increased current flow produces a voltage drop across thecurrent sensing resistor R This current is allowed to build up until thevoltage drop across the current sensing resistor R approximately equalsthe potential applied at the base of transistor Q at which time the basedrive on the transistor Q will be insufficient to support additionalcurrent flow. When this occurs, the voltage across the current sensingresistor R will in effect track the voltage applied at the baseelectrode of transistor Q Also, transistor Q, has a voltage drop that issubstantially equal to the difference between the supply voltage andvoltage developed across the transformer T If lamp 99 tries to draw morecurrent than the value corresponding to the limited voltage developedacross the current sensing resistor R the transistor impedance increasesand the voltage drop across the collector and emitter electrodes oftransistor Q will increase thereby causing the voltage applied to theprimary winding P to decrease. Similarly, when the lamp currentdecreases, this voltage drop will decrease and cause the voltage acrossthe secondary winding S to increase. In this manner, the current to lamp99 is dynamically controlled by the varying impedance introduced by thetransistor Q If the secondary winding S is short circuited, the currentin the circuit is still effectively limited by the voltage drop acrossthe resistor R and by the voltage available at the base electrode oftransistor Q In this case, the voltage developed across transformer T iszero. "On the other hand, when output leads 104, 105 are open circuited,the full rectified output of the power source is made available acrossthe primary winding P to provide the maximum voltage across thesecondary winding S When the voltage across the primary winding Preverses, it will be understood that the lower end of the secondarywinding S is now negative with respect to the upper end and a negativevoltage now appears at the base electrode of transistor Q During thisalternation of the power source, primary winding P provides the drivingvoltage for transformer T and the loop which includes transistor Qcurrent sensing resistor R and the primary winding P come into play. Inthe same manner as during the previous alternation of the power supply,a decreasing current flow through the secondary winding S has the effectof lowering the resistance reflected into the primary winding P andthereby causing more current to be supplied thereto from the powersource through bridge rectifier 109. As this current flow in the loopincreases, the voltage drop across the current sensing resistor Rincreases. The current is allowed to build up until the voltage dropacross the current sensing resistor R is nearly equal to the potentialat the base electrode of transistor Q At this point, the base drive ontransistor Q will be insuflicient to support additional current flowthrough the transistor Q If the lamp circuit now attempts to draw morecurrent, the voltage drop across the collector and emitter electrode oftransistor Q will increase. Thus, the voltage across the primary windingP will decrease, and lamp operating voltage across the secondary windingS decreases. In this way, the lamp current is limited by the varyingimpedance of the transistOI Q10.

Although, in the above described exemplification of the invention, thefeedback signal was an alternating signal having a substantiallysinusoidal waveshape, and in phase with the power source, it will beappreciated that other signals of different waveshapes may be providedto drive the variable impedance bridge circuit of the apparatus 5 100.For example, if the voltage across the transformer secondary winding Sis fed back to the feedback leads 106 and 107, the signal will have thewaveshape of the lamp voltage, and the lamp current waveshape will becontrolled to correspond with the lamp voltage waveshape. With such anarrangement, it will be apparent that unity lamp power factor can beachieved. Further, it will be appreciated that with the variableimpedance bridge network arrangement shown, any desired waveshape of thelamp current can be obtained since the apparatus 100 will essentiallyprovide a current in the secondary winding S having a waveshapecorresponding to the waveshape of the voltage signal applied across theprimary winding P14.

From the foregoing description of the invention, it will be apparentthat the ballasting action for one or more fluorescent lamps is providedby a variable impedance network. This network introduces aninstantaneously variable impedance which may regulate lamp currentindirectly. An important advantage of the invention is that the variableimpedance network makes it possible to minimize losses in the circuitthat would otherwise result if a linear resistor were used as theballasting element. Also, the variable impedance network arrangementmakes it possible to design an apparatus for operating electricdischarge lamps with a smaller difference between the supply voltage andthe operating voltage of the lamp than would be the case if conventionalballasting elements were employed in the circuit to provide the currentlimiting action for the electric discharge lamps. Further, the variableimpedance network of the invention is readily adaptable to control by asignal responsive to the lamp operating condition. A signal sensing anoperating condition or a signal from an independent source may beemployed to control the waveshape of the lamp current.

Although a variable impedance network utilizing a bridge has beenemployed in the exemplification of my invention, it will be apparent tothose skilled in the art that variable impedance networks employingbilateral semiconductor devices or unidirectional devices in an inversearrangement may be used in the practice of the invention. It will beunderstood that the specific exemplifications of the invention which Ihave described herein are intended for illustrative purposes only andthat many modifications may be made. It is, therefore, intended by theappended claims to cover all such modifications that fall within thetrue spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a ballast apparatus for operating at least one electric dischargelamp from an alternating power source, said apparatus having a pair ofinput leads for connection with the alternating power source and a pairof output leads for connection to the electric discharge lamp, avariable impedance network comprising a full wave rectifying meanshaving a pair of input terminals and a pair of output terminals, saidinput terminals being connected in circuit with the input leads forconnection to the alternating power source, a pair of transistors, eachof said transistors having a collector, an emitter and a base electrode,a transformer having a primary winding and a secondary windinginductively coupled therewith, one of said windings being connected incircuit with at least one of said output leads, the other of saidwindings having the ends thereof connected in circuit with the collectorelectrodes of said transistors, said other winding having a tapconnected in circuit with one of the output terminals of said full waverectifying means, circuit means including a resistor connecting theother of said output terminals of said full wave rectifying means incircuit with the emitter electrodes of said transistors, a bias supplymeans connected in circuit with the base electrodes of said transistors,said transistors being driven by said bias supply means to provide avariable impedance in the primary circuit of said transformer in eachhalf cycle of the alternating power source whereby the current providedat said output leads is regulated.

2. The apparatus as set forth in claim 1 wherein said bias supply meansincludes a transformer having a primary winding and a secondary winding,the ends of said secondary winding being connected in circuit with thebase electrodes of said transistors, said secondary winding having a tapconnected in circuit with said other output terminal of said full waverectifying means, said primary winding having leads for connection incircuit with a feedback signal source whereby the waveform of thecurrent supplied at said output leads corresponds to the waveform of thefeedback signal supplied from the feedback signal source.

3. Ballast apparatus comprising a pair of input leads for connection toan alternating current power source, a pair of output leads forconnection to an electric discharge lamp, a rectifier having inputterminals and output terminals, said input terminals being connected incircuit with said input leads for connection to said power source, atleast one transistor having an input electrode and output electrodes, atransformer having a primary winding and a secondary winding coupledthereto, means coupling said secondary winding in circuit with saidoutput leads for connection to said lamp, means coupling said primarywinding to one of said output electrodes of said transistor, meanscoupling another point on said primary winding to one of said rectifieroutput terminals, means coupling an impedance between the other of saidrectifier output terminals and the other of said output electrodes ofsaid transistor, and means coupled to said input electrode of saidtransistor for alternately supplying signals thereto to render saidtransistors alternately conductive and nonconductive.

4. Ballast apparatus comprising a pair of input leads for connection toan alternating current power source, a pair of output leads forconnection to an electric discharge lamp, a full wave rectifier having apair of input terminals and a pair of output terminals, means couplingsaid rectifier input terminals in circuit with said input leads forconnection to said power source, a pair of transistors each having aninput electrode and output electrodes, a transformer having a tappedprimary winding and a secondary winding coupled thereto, means couplingsaid secondary winding in circuit with said output leads for connectionto said lamp, means coupling said primary winding to corresponding onesof said output electrodes of each of said transistors, means couplingsaid tap on said primary winding to one of said rectifier outputterminals, means coupling an impedance between the other of saidrectifier output terminals and corresponding other output electrodes ofeach of said transistors, and means coupled to said input electrodes ofeach of said transistors for alternately supplying a signal thereto toalt rnately render one of said transistors conductive and the other ofsaid transistors nonconductive.

5. A ballast apparatus for operating at least one electric dischargelamp from an alternating power source, said apparatus comprising a pairof input leads for connection with the alternating power source and apair of output leads for connection to the electric discharge lamp, avariable impedance network for controlling the current supplied by saidoutput leads, said network comprising a full wave rectifying meanshaving a pair of input terminals and a pair of output terminals, saidinput terminals being connected in circuit with the input leads forconnection to the alternating power source, a pair of transistors, eachof said transistors having a collector, an emitter, and a baseelectrode, a first transformer having a primary winding and a secondarywinding inductively coupled therewith, one of said windings beingconnected in circuit with at least one of said output leads, the otherof said windings having the ends thereof connected directly to thecollector electrodes of said transistors, said other winding having atap connected in circuit only with one of the output terminals of saidfull wave rectifying means, circuit means including a current sensingresistor connecting the other of said output terminals of said full waverectifying means in circuit with the emitter electrodes of saidtransistors, a bias supply means for driving said transistors, said biassupply means comprising a bias signal transformer isolated from saidfirst transformer and having a primary winding and a secondary windingwith the ends thereof connected to the base electrodes of saidtransistors, and circuit means connecting the center tap of saidsecondary winding directly with said other one of the output terminalsof said full wave rectifying means, said transistors in response to thevoltage drop across said current sensing resistor providing a variableimpedance in the primary circuit of said transformer in each half cycleof the alternating power source, whereby the current provided at saidoutput leads is regulated.

References Cited UNITED STATES PATENTS 5/1956 Pearlman 331--114 2/1960Greene et a1 3l5-138 US. Cl. X.R. 315281, 282

